1. Field of the Invention
The present invention relates generally to power calculations for intraocular lenses (IOLs) and more particularly, is directed to a customized polychromatic or monochromatic ray tracing-based IOL power calculation that includes subject eye measurement data, resulting in an improved IOL power calculation compared to prior regression-based IOL power calculations, particularly for patients that previously underwent an ablative form of keratorefractive surgery, such as lasik.
2. Description of the Background
Intraocular Lenses (IOLs) may be used for restoring visual performance after removal of the natural crystalline lens of an eye, such as in cataract surgery. Because the IOL geometry may be selected, it is desirable to select an IOL geometry that will most nearly achieve emmetropia when the IOL is implanted. The term emmetropia, and variations thereof, is used herein and within the art to indicate a state of vision in which an object at effectively infinite distance from the subject is in sharp focus on the subject's retina.
The power necessary for the IOL to provide emmetropia has typically been calculated using classical regression theory. For example, the Saunders, Retzlaff, and Kraff formula (SRK) is a regression formula derived from clinical data to indicate the optimal power for an IOL. The SRK regression formula is:P=A−2.5*AXL−0.9*K where P is the IOL power, A is the lens constant, AXL is the axial length in millimeters of the subject eye, and K is the average corneal power in diopters (D). Because the SRK regression formula was derived from historical clinical data, it necessarily provides the best result when the subject eye has dimensions similar to the most common eye dimensions that were included in the clinical data from which the formula was derived. In particular, the SRK typically underestimates the necessary IOL power to obtain emmetropia for short eyes, and overestimates the IOL power necessary for long eyes. That is, use of the SRK formula may lead to the selection of an IOL power for long eyes that is too strong, and an IOL power for short eyes that is too weak.
In order to remedy these shortcomings of SRK, other regression formulas were developed that incorporate certain refinements, but which are still based on the analysis of historical clinical data. In the SRK II regression for example, an additional constant F is included to adjust the IOL power calculation based on the length of the eye. More particularly, the SRK II formula is:P=A−2.5*AXL−0.9*K+F wherein F is a known value, which may be equal to +3 D at less than 20 millimeters of axial length (AXL), +2 D at 20 to 20.9 millimeters, +1 D at 21 to 21.9 millimeters, OD at 22 to 22.5 millimeters, and −0.5 D at greater than 24.5 millimeters. By way of example, if SRK yields an IOL power of +32 D, SRK II may yield an IOL power of +35 D (+32 D+3 D=+35 D) if the patient's AXL is less than 20 mm.
An additional regression method, developed in an effort to address the shortcomings of SRK and SRK II, is the SRK/T method. In the SRK/T, the approach is different. An empirical regression method is used for prediction of the IOL position after the surgery. Once that position is determined, the preferred power for an IOL to be implanted is calculated by simple paraxial optics, taking into account that the eye can be modeled under this approximation as a two lens system (cornea+IOL) focusing an image on the retina. This approach is based on Fyodorov's theoretical formula.
Thus, there currently exists a large catalog of formulas for calculating IOL power, such as the aforementioned SRK/T, as well as, Haigis, Hoffer Q, and Holladay 1 and 2, for example. It is well known that these formulas do not provide accurate predictions for all preoperative refractive states. While a good prediction to achieve emmetropia after surgery may be obtained for patients that were emmetropes or close to emmetropia prior to cataract surgery, errors arise for extreme myopes or hyperopes. Such deviations for “non-normal” eyes are not unexpected, since the regression formulas were derived from analyses of historical data, and therefore necessarily make recommendations that are most favorable for “normal eyes.”
For example, FIG. 1 illustrates the variations in IOL power recommendations provided by different regression calculation methods for a set of patients, each indicated by a patient number. As illustrated, the differences in IOL power recommended by the various methods for a particular patient are small when the patient has “normal eyes,” indicated by the shaded rectangular area 100. However, the differences become more extreme for progressively more myopic or hyperopic eyes, particularly for eyes having a recommended IOL power less than about 17 D and greater than about 24 D.
Optical aberrations are another important point that is not taken into account in regular regression formulas. There are two main types of aberrations, monochromatic and polychromatic. The former may be related with either the cornea or the IOL, while the latter is mainly related to the IOL material. Every specific eye has a different pattern of monochromatic aberrations and therefore their impact cannot be averaged by optimized constants or regression coefficients. In addition, due to the paraxial nature of regular regression formulas, the impact of optical aberrations is totally neglected.
Eyes that have undergone refractive surgery, such as radial keratotomy (RK), photorefractive keratectomy (PRK), laser-assisted in situ keratomileusis (lasik), or the like, are another example of eyes that are out of the “normal” range, at least in part because the corneal power of the post operative eye has been modified by the refractive surgery. Thus, it is well known that regression formulas typically do not provide proper IOL power for those patients. Other factors that complicate predicting an optimal IOL power for such patients include the corneal power (K), which may be incorrectly measured by topographers or keratometers after a refractive surgery procedure. In addition, the expected relationship between the anterior and posterior corneal radius may be modified by refractive surgery, which renders the corneal equivalent refractive index calculated for normal patients inapplicable, leading to an inaccurate total corneal power.
Moreover, it has been widely reported that standard ablative forms of keratorefractive surgery, such as lasik, may result in a larger than normal proportion of corneal aberrations. Such aberrations may affect the IOL power predicted by the regression formulas, which do not consider such aberrations, due to their paraxial nature.
Thus, there is a need for a system and method that provides improved accuracy in predicting optimal IOL power for patients whose eyes are inside as well as outside of the normal range, with respect to axial lengths, amount of aberrations or preoperative state.